Aircraft engine noise is a significant problem in high population areas and noise-controlled environments. The noise is generally composed of contributions from various source mechanisms in the aircraft, with fan noise typically being a dominant component of engine noise at take-off and landing. Fan noise is generated at the fan of the aircraft engine, propagates through the engine intake and exhaust duct, and is then radiated to the outside environment. Acoustic liners are known to be applied on the internal walls of the engine's casing and hub to attenuate the fan noise propagating through the engine ducts. Typical acoustic liners for engines are either a single degree of freedom (SDOF) liner, or a two degree of freedom (2DOF) liner, sometimes referred to as a double degree of freedom (DDOF) liner. Acoustic liners also may be applied to other portions of the engine to attenuate noise from other engine components. Further, the principles of acoustic liners may apply generally to noise attenuation structures for other applications.
Usually, SDOF liners are formed of a porous facing sheet backed by a single layer of cellular separator such as honeycomb cells, which itself is backed by a solid backing plate that is substantially impervious to higher frequency noise transmission. 2DOF liners, on the other hand, are formed of two cellular layers between the porous facing sheet and the solid backing plate, with the two cellular layers separated by a porous septum sheet. The acoustic performance of both SDOF and 2DOF liners is strongly dependent on the depth of the cells in each honeycomb layer, where the cell depth controls the internal volume of the cell that is available for acoustic resonance. The additional layer of the 2DOF liner allows noise suppression of at least one other main frequency than the SDOF liner. However, the additional layer of the 2DOF liner significantly increases the weight of and cost to produce the liner and also requires more room to accommodate the additional layer, which increases the thickness of the liner.
At least some known SDOF honeycomb acoustic liners attempt to achieve the multiple frequency advantages of the 2DOF liner in an SDOF construction by forming individual cells within the core layer to have variable depths from the perforate facing sheet, thereby creating different resonant cavity volumes within the same SDOF layer. However, this variable depth construction approach is very costly because of the complexity of actively modifying cells depths for individual cells. Further, some acoustic liners attempt to increase noise attenuation by providing acoustic communication between the cells of the core layer. Nevertheless, such attempts provide a fixed porosity or percent open area of the cell walls, which at least fails to accommodate changes in engine noise (e.g., changes in frequency and/or axial wavenumber) based on engine operating conditions.
Accordingly, improved noise attenuation structures, such as structures that allow non-locally reactive noise attenuation, would be desirable. For example, noise attenuation structures, such as acoustic liners for gas turbine engines, that include features for active variable depth control of the cells would be beneficial. Further, noise attenuation structures that include features for active control of the core layer porosity would be useful.